Live attenuated simian immunodeficiency virus (SIV) has been shown to be the best candidate vaccine for inhibiting SIV infection. However, the ability of live attenuated SIV to cause AIDS-like disease in newborn macaques raises concern over the safety of a live virus as a vaccine. Thus, in an effort to improve on the live attenuated SIV as a vaccine, we generated a SIVmac239 construct which encodes sequence elements that should facilitate the excision of an integrated SIV proviral genome. During the past year, we evaluated the ability of this recombinant SIVmac239 construct to replicate in transformed CD4+ T cells and in macaque peripheral blood mononuclear cells (PBMCs). From these in vitro studies we conclude 1) That the recombinant SIVmac239 containing loxP sequence elements and the cre-recombinase gene is capable of replication in both cultured T cells and in primary rhesus PBMC. These results suggest that the recombinant SIV should be capable of infection and replication in rhesus macaques experimentally inoculated. Extensive evaluation of unique live attenuated SIV in rhesus macaques may provide the means for creating a safe and effective vaccine to prevent the progression of AIDS.